This invention generally relates to mixtures resulting from the dimerization of norbornadiene. In particular, the invention relates to a mixture having a high concentration of a monoolefinic hexacyclic hydrocarbon known by the systematic chemical name of exo-endo stereoisomer of hexacyclo (7.2.1.0.sup.2,8.1.sup.3,7.1.sup.5,13.1.sup.4,6)tetradec-10-ene (also designated as hexacyclo [9.2.1.0.sup.2,10.0.sup.3,8.0.sup.4,6.0.sup.5,9 ]tetradec-12-ene). The stereoisomer results from the catalytic dimerization of norbornadiene which is a C.sub.7 H.sub.8 bicyclic, diolefinic hydrocarbon. More particularly, the invention relates to a mixture containing a high concentration of the exo-endo form of the hexacyclic dimer of norbornadiene and some endo-endo hexacyclic dimer of norbornadiene. Both of the dimers are C.sub.14 H.sub.16, six-ring monoolefinic hydrocarbons. Also, the invention relates to a mixture of the foregoing which has been hydrogenated to convert the monoolefinic hexacyclic hydrocarbon to a completely saturated hexacyclic hydrocarbon. Hydrogenation of a monoolefinic hexacyclic dimer to the saturated dimer improves stability of the product towards oxidation thereby enhancing its utility as a high energy fuel. A mixture of completely saturated exo-endo and endo-endo hexacyclic dimers has a utility as a component of a high energy fuel.
An object of present invention is to provide a composition which has a maximum concentration of hexacyclic norbornadiene dimers and a minimal concentration of pentacyclic norbornadiene dimers and other compounds. Also the composition can be used as a component of high energy fuel for use in either jet or rocket propulsion. Jet propulsion includes a jet engine which can be used for missile, plane and other applications and includes the three basic types, i.e., ramjet, turbo-jet and pulse jet. The term jet generally refers to a device requiring air whereas rocket generally refers to a device containing its own oxygen or oxidizing agent.
Another object of present invention is to provide a novel method for preparing the foregoing composition. Still another object is the dimerization of norbornadiene at both excellent selectivity and conversion to the exo-endo form of the four possible stereoisomeric hexacyclic dimers.
Norbornadiene is also known as bicyclo-[2.2.1) heptadiene-2,5. A method of preparation is disclosed in U.S. Pat. No. 2,875,256, issued Feb. 24, 1959. Norbornadiene will be referred to as NBD hereinafter. NBD can be represented by either one of the following structural foamulas: ##STR1##
Dimerization of NBD is disclosed in U.S. Pat. No. 3,377,398, issued Apr. 9, 1968. The disclosed process results in the production of various dimer mixtures. The process therein involves the use of an iron catalyst system, e.g., ferric acetylacetonate and triethylaluminum, and a temperature above 140.degree. C. The product of said method is a mixture which includes both monoolefinic hexacyclic and diolefinic pentacyclic dimers.
U.S. Pat. No. 3,282,663, issued Nov. 1, 1966, also discloses the dimerization of NBD to pentacyclic and hexacyclic dimers. In one example, ferric acetylacetonate and triethylaluminum is the catalyst. One of the dimers reported therein, i.e., Dimer III, has been identified as the endo-endo stereoisomer of the hexacyclic dimer of norbornadiene.
U.S. Pat. No. 3,326,992, issued June 20, 1967, discloses the partial hydrogenation of NBD dimer mixtures.
German patent publications Nos. 215332 and 2153314 disclose catalytic complexes of rhodium and iridium. Publication date for both of the aforementioned publications is Apr. 27, 1972. Both are West German publications.
A catalytic reaction between NBD and butadiene is disclosed in an article in the Journal of Organic Chemistry, Jan., 1970, Vol. 35, title, "Catalytic Norbornadiene-Butadiene and Norbornadiene-1,1-Dimethylallene Codimerization", by A. Greco et al., pages 271-274. One of the disclosed catalysts is a three component system of tris(acetylacetonate)iron-AlEt.sub.2 Cl-bis (diphenylophosphine)ethane. AlEt.sub.2 Cl refers to diethylaluminum chloride. One of the dimers reported therein, i.e. FIG. 1e, has been identified as the exo-exo stereoisomer of the hexacyclic dimer of norbornadiene.
Also, a catalytic reaction of NBD is disclosed in an article in The Journal of the American Chemical Society, Vol. 94, July 26, 1972, starting page 5446, titled "Dimerization and Trimerization of Norbornadiene by Soluble Rhodium Catalyst", by Nancy Acton et al. Rhodium catalysts, such as [(C.sub.6 H.sub.5).sub.3 P].sub.3 RhCl, are disclosed. NBD dimers disclosed include the exo-endo and endo-endo isomeric forms.
Another article, "Catalysis of Cycloaddition Reaction by Rhodium on Carbon", by J. J. Mrowca et al, Journal of the American Chemical Society, 88:17, Sept. 5, 1966, pages 4012-4015, discloses the use of rhodium on carbon to dimerize NBD. The resulting products contain a major amount of the exo-endo stereoisomer of the hexacyclic dimer of NBD and a minor amount of the endo-endo dimer.
As the previous discussion indicates, more than one NBD dimer is possible. G. N. Schzauzer, in his review "On Transition Metal-Catalyzed Reactions of Norbornadiene and the Concept of a Complex Multicenter Processes" in Advances on Catalysis 18, 373 (1968) Acad. Press, describes the fourteen theoretically possible dimers of NBD. The possible dimers, grouped according to the number of their carbocyclic rings, are shown in accompanying drawing. Any and each of the dimers shown in the drawing have different physical and chemical properties.
Thus, a specific synthesis problem in the dimerization of NBD, as can be visualized from the number of possible isomers, is to obtain both excellent selectivity and conversion to a desired isomer.
The advantages of the present invention are as follows. The production of exo-endo and endo-endo hexacyclic dimers are favored while the production of pentacyclics is minimized. The latter are not desirable as high energy fuels or components thereof because of their high melting points and separation of pentacyclic dimers from the hexacyclic dimers is commercially not feasible. On the other hand, a mixture of exo-endo and endo-endo hexacyclic dimers can be readily separated from small amounts of unreacted feed and other products, particularly higher boiling polymers such as trimers. Thus, a finished product can be obtained consisting essentially of the exo-endo and endo-endo material. Hydrogenation of the foregoing material provides a mixture which can be used as a component for high energy, high density fuel.